Spectrum Sensing for Powering Power Line Communications

Transcription

1 Spectrum Sensing for Powering Power Line Communications Laryssa R. Amado, Eduarda S. C. Losqui, Fabricio P. V. de Campos, Alvaro Augusto M. de Medeiros, and Moises V. Ribeiro Abstract The use of electric grids for communication purposes may interfere with radio communication services. To offer new directions to avoid electromagnetic interference that reduces the effectiveness of electric power grids as a green and lowcost communication channel, this paper discusses and introduces spectrum sensing techniques for cognitive power line communication (PLC) systems. Additionally, a measurement campaign for monitoring the radio spectrum in the range from 1.7 to 1 MHz is discussed as well as attained results with the spectrum sensing technique. The numerical results indicate the proposed spectrum sensing technique can offer good performance in a cognitive PLC systems. Keywords power line communication, spectrum sensing, signal processing. I. ITRODUCTIO Among the available technologies, the power line communication (PLC) has emerged as an interesting candidate for what is being called smart grid communications (SGC) [1]. However, the electric power grids were not conceived for the high-frequency data communication. Also, they are not shielded and their electromagnetic radiation may interfere with radio communication services. The potential disturbances yielded by PLC signals that could result in interference in other telecommunication systems operating in the same bandwidth, near and far from the power line cable were discussed, investigated, and measured [2]. To overcome this problem, several mitigation techniques have been proposed and introduced in some regulation and standardization documents to protect radio frequencies. The majority of these techniques are based on fixed, dynamic, and prefixed notching techniques [3], [4]. However, it is well known that the usability of the spectrum by primary users are time and spatial dependent and the scarcity of the spectrum for data communication, mainly for SGC, is pushing forward the introduction of cognitive concepts to improve the performance of PLC systems [3] [6]. The use of some spectrum monitoring techniques to detect the presence of primary users was analyzed based on spectrum analyzer measurements, performed on the power line cable Laryssa R. Amado, Eduarda S. C. Losqui, Alvaro Augusto M. de Medeiros and Moises V. Ribeiro are with Department of Electrical Engineering, Federal University of Juiz de Fora Juiz de Fora, MG, , Brazil, {laryssa.amado, eduarda.losqui, alvaro, Fabrício P. V. de Campos is with Electrical Engineering Program,COPPE/Federal University of Rio de Janeiro 6854, , Rio de Janeiro, RJ, Brazil paper was supported in part by FAPEMIG, CPq, CAPES, CAPES-PPD, FIEP, P&D AEEL, and IERGE. through a capacitive coupler or on an antenna [4], [7], [8]. Cooperative spectrum monitoring techniques are adequate to detect radio frequencies when the signal is measured directly on the power line cable [3]. Although it offers a new perspective for improving the spectrum usability for PLC systems, several questions deserve a deeper analysis for powering the PLC devices with cognitive features that result in a SGC infrastructure that guarantees QoS for smart grid, as well as electric power systems applications in the high-, medium-, and low-voltage electric grids (outdoor and indoor). This paper aims to discuss and analyze a spectrum sensing technique for monitoring the frequency bandwidth between 1.7 and 1 MHz. In addition, this paper presents some results related to an initial measurement campaign, which was carried out in Brazil, to monitor the spectrum in the aforesaid frequency bandwidth with a setup that makes use of the spectrum sensing technique that analyzes the measured data. The results based on the measurements show that: i) the performance of the spectrum sensing technique based on the choice of transform depend upon the length of the window; ii) differently from what was pointed out [3], both signals measured with a antenna or a coupler connected to the electric grid outlet can provide detection rate higher than 99% if the suggested technique and setup are considered. II. PROBLEM FORMULATIO Let a random and monitored signal be expressed by x(t) = s(t)+v(t) = s k (t)+v(t), (1) where s k (t) R is the signal generated by the k-th spectrum user (analog or digital); K is the number spectrum users, v(t) R is the additive noise; x(t) is a time domain signal with the presence of noise. The acquisition of the signal x(t) for spectrum sensing can be made by different sensors (transductor associated to an A/D device). Supposing that there are M different types of sensors, then, the use of a m-th sensor to acquire the signal x(t), results in x = P m(x(t)) = P m(s(t)) +P m(v(t)) = s+v, (2) where P = {P 1, P 2,...,P M } defines a set of M sensors; x R 1, s R 1 and v R 1 are vectors with coefficients, which were obtained from the signals x(t), s(t) and v(t), respectively. Vector x is constituted of samples from the signal x(t), which were taken at rate f s = 2B, where B is the signal bandwidth, whose frequencies extend from to B Hz, i.e. f [,B).

2 A linear transformation is applied to the vector x to explicitly show the spectral signal content of s k, k = 1, 2,..., K, in order to sense the spectrum. Therefore, the transformed signal is expressed by Fig. 1. x G(.) X E(.) r d D(.) λ Block diagram of the detection system. X = A px = S+V = S k +V (3) where X C 1, S k = A ps k C 1, V k = A pv C 1, and A = {A 1, A 2,...,A P } represents a set of P spectrum transform techniques, such that A p C. ote that S k = [ T k,a S k(l a,k )... S k (L a,k +L k 1) T k,b ]T, k,a and k,b are L k,a 1 and L k,b 1 vectors constituted by zeros, and L k,a +L k,b +L k =, k Z. By diving the vector X into L q-length window, then we have X d,lq = S d,lq +V d,lq = S k,d,lq +V d,lq (4) where X d,lq = [X((d 1)L q) X((d 1)L q +1)... X(L q 1)] T, is a vector constituted by L q consecutive coefficients of the vector X. Similarly, S d,lq = [S((d 1)L q) S((d 1)L q + 1)... S(L q 1)] T and V d,lq = [V((d 1)L q) V((d 1)L q + 1)... V(L q 1)] T, where q = 1, 2,..., Q. Let L = {L 1, L 2,..., L Q } be a set of vector lengths. Hence, the spectrum sensing problem on PLC systems can be formulated as two simple hypothesis X d,lq = V d,lq, H, S k,d,lq +V d,lq, H 1 (5) where H and H 1 are the hypothesis associated to the absence and presence, respectively, of the k-th data communication system (narrowband or broadband). Consider a R-length feature vector, r d R R 1, extracted from X d,lq aiming to reduce its dimensionality. Supposing that different extraction and selection techniques are used to define the vector r d, then a set of features can be represented by Ψ = Ψ 1,Ψ 2,...,Ψ G. Assuming that P D (detection probability) and P F (false alarm probability) are expressed, respectively, by P D = P r(r d > λ H ) and P F = P r(r d > λ H 1 ), where P r(.) are the probabilities and λ is the detection threshold. The spectrum sensing problem for a PLC system can be formulated as a multi-objective problem, which can be expressed as maxp D,min{P F, C, τ}, (A, L, D, P, Ψ, C, T ) where A = {A 1, A 2,...,A P } is the set of transforms; L = {L 1, L 2,..., L q} is the set of window lengths; D = {D 1, D 2,..., D U } is the set of detection techniques; P = {P 1, P 2,..., P M } is the set of sensors; Ψ = Ψ 1,Ψ 2,...,Ψ G is the set of features; C = {C 1, C 2,..., C J } is the set of computational complexities related to each choice and T = {τ 1, τ 2,..., τ V } is the set of time intervals in which spectrum sensing should be used. III. THE SPECTRUM SESIG TECHIQUE The block diagram of the detection technique used for spectrum sensing is shown in Figure 1. Each one of the blocks can be described as: (6) Block G(.): it represents the signal transformation applied to the vector x. Due to the necessity of analyzing the frequency spectrum of this signal, the following transforms were assessed for use: DFT (Discrete Fourier Transform), DHT (Discrete Hartley Transform), DCT (Discrete Cosine Transform), DST (Discrete Sine Transform) and MCLT (Modulated Complex Lapped Transform). Block E(.): it represents the operator that first defines the vector X d,lq from vector X, and then extracts the feature vector r d R R 1, such that R L q. Block D(.): it implements the algorithm that provides results into states whether there is or there is not a radio signal in the vector X d,lq. Based on different characteristics of the signals and noise located at different parts of the selected spectrum, we suggest that the frequency bandwidth between 1.75 and 1 MHz should be divided into three sub-bands, see Figure 2-A, for spectrum monitoring application. The rationale is that those distinct frequency bands show different measured background noise levels, which can reduce the performance of the designed spectrum detection technique if only one configuration is designed to detect signal in all frequency bands. Fig. 2. The suggested spectrum division for sensing applications related to PLC system. A. Transforms For spectrum sensing applications, it is necessary to apply transforms to the signal, so that its spectral content becomes apparent. Therefore, any transform that provides the time frequency representation of the signal [9] can be included in the set A. In this context, this Section describes the following transforms: DFT, DHT, DCT, DST and MCLT, each of them were chosen because they can reveal spectrum information of the signal. It is assumed that finite length sequence is {x[n]} 1, such that x[n] R. The DFT is known as the standard technique for spectrum sensing. The DFT is expressed by 1 X[k] = x[n]w kn, k 1, (7) where W r = exp{2πr/}. The DHT [1] can be given by 1 ( ) 2π X[k] = x[n]cas kn, k 1, (8)

3 where cas(x) = cos(x) + sin(x). The DCT is expressed by [1] X[k] = α(n)x[n] cos where 1 α(n) = 2 The DST IV is given by X[k] = 1 α(n)x[n] sin ( (2k+1)nπ 2 ), k =, 1,..., 1 n = or 1 otherwise [ ] (2k+1)nπ, 2 k =, 1,..., 1 where 1 α(n) = n = ou 1. 2 otherwise The MCLT [1] is given by. (9) (1) 2 1 X[k] = x[n]p[n,k], k =, 1,..., 1, (11) where p[n,k] is a basic analysis function, defined by p[n,k] = p c[n,k] jp s[n,k], j = 1, and (( ) 2 (k ) p C [n,k] = h[n]cos n π 2) (( ) 2 (k ) p S [n,k] = h[n]sin (12) n π 2), in which h[n] is the modulated impulse response of the filter B. Feature Extraction Technique Feature extraction techniques provide a large set of features related to the a vector. A premium is placed on extracting relevant features that are somewhat invariant to the change of the vector X d,lq in order to perform the detection. There are several kinds of feature extraction techniques for both real and complex sequences. The most common feature applied for spectrum sensing is the energy of signal due to its computational complexity and the lack of necessity of previous information about the transmitted signal [11]. In this paper, besides energy of signal, we focus on HOS- (higher order statistics), skewness-, and kurtosis-based features because they have been effective in other applications demanding detection technique [12]. Due to paper size restrictions, the expressions and details for each feature are not presented here. A summary of each of them can be found in [6]. C. Feature Selection Technique Feature selection is a process that selects the most significant features of a set, aiming to facilitate the detection process through the use of the least possible number of features. This stage is crucial to the detection process, once it enables a better performance of this process and the design of a simpler detector. One of the most used and simple feature extraction technique that presents reasonable results is the Fisher s Discriminant Ratio(FDR) [13]. Again, due to paper size restrictions, expressions of the FDR were suppressed. A more detailed presentation of the use of FDR on spectrum sensing purposes can be found in [6]. D. Detection Technique After reducing the dimensionality of the vector X d,lq, we ought to test whether the chosen detector is suitable to this problem or not. There are several detection techniques based on detection theory. However, the analysis of the histogram of the extracted feature vector reveals how complicated it can be to fit into a distribution model. In fact, although the number of measurement are representative, we believe that much more measurements need to be carried out to provide a representative set of monitored signals, from which the distribution of feature vector can be obtained. Therefore, in this paper, we have chosen the Multi-Layer Perceptron eural etwork (MLP) and a training procedure highlighted in [14], because it had provided good performance in detection and classification problems related to other applications. Due to limitation on number of pages, the expressions for the MPL are not presented here, but they can be found in [6]. IV. THE SPECTRUM MEASUREMET CAMPAIG The measured data were acquired by the acquisition board Gage Razor CompuScope 1642 [15], which consists of several analog-to-digital converters of 16 bits of vertical resolution with maximum sampling rate of 2 MS/s by channel. Two kind of sensors were used to acquire the spectrum signal: i) an omnidirectional antenna with bandwidth raging from 1 MHz up to 1 MHz and impedance equals 5 Ω, and ii) a capacitive coupler (PREMO P ) [16], which introduces an increasing attenuation when the frequency goes beyond 3 MHz and owns a high-pass filter with cutoff frequency f c =1.7 MHz. Two input channels of the acquisition board were used to measure the signal with the antenna and capacitive coupler simultaneously. The simultaneous acquisition of the spectrum signal with the aforementioned sensors (transducers) aims to provide an answer to the investigation question #1. The measurements were made at 16 different locations in the city of Juiz de Fora, during the morning, afternoon and evening, to capture different influences, since it is known that the spectrum influences vary with time and space as well. In each period of time, 1 measurements were taken every 6 seconds, an empirically determined interval. On total, 48 measurements were taken and each measure is constituted by 8192 samples. The places are close to airport, army facilities and urban areas (home and building of low- and medium-income habitants). The time-dependence of the spectrum can be observed in Figures 3, 4 and 5. ote that during the morning a very specific kind of source of disturbance yielded a noise that corrupted the spectrum. Also, only during the evening hours the army made use of the frequency bandwidth around 4 MHz for data communications, what reveals that the use of spectrum sensing technique can maximize the usability of these frequency bandwidth. Additionally, it could be verified that several bands allocated to some radio communication systems are scarcely used.

4 happens for the DST, when L q =32, as shown in Table III. The window size that most frequently led to the best result for each transform was L q =32. In Table IV, the highest detection rate is related to the DCT and L q = 16. However, the window sizes that show the best results for each transform are L q =128 and L q = TABELA I DETECTIO RATES FOR THE MEASUREMETS MADE USIG THE ATEA (%) - SUB-BAD#1. Fig. 3. The amplitude spectrum, based on DFT, of a signal measured with the antenna in the downtown in the morning DFT MCLT DHT DCT DST Fig. 4. The amplitude spectrum, based on DFT, of a signal measured with the antenna in downtown in the afternoon. TABELA II SUB-BAD#1. DFT MCLT DHT DCT DST TABELA III DETECTIO RATES FOR THE MEASUREMETS MADE O THE ATEA (%) - SUB-BAD#2. DFT MCLT DHT DCT DST Fig. 5. The amplitude spectrum, based on DFT, of a signal measured with the antenna in downtown in the evening. A. Measured Data V. UMERICAL RESULTS The following considerations were taken into account for measuring the spectrum signals as well as to design the spectrum sensing technique: B =1 MHz, =8192, and R =3. Tables I, III, and V show the results obtained for the measurements made on the antenna, while Tables II, IV, and VI display the results for the measurements made on the cable, using the capacitive coupler. The detection rates for signals from 1.75 MHz up to 25 MHz measured on the antenna and coupler are shown in Table I and II. It can be seen that on the average the DST and DCT provide the best performance in terms of detection rate while the DFT the worst one. Overall, the best performances were attained when L q =128 and L q =256. The interval from 25 MHz to 75 MHz, for the data measured on the antenna and coupler, the highest detection rate, 99.27%, TABELA IV SUB-BAD#2. DFT MCLT DHT DCT DST The last part of the spectrum detection rate for the antenna measured data is shown in Table V, and the highest rate P D =1% was calculated for the DCT transform, when L q =256. The last part of the spectrum of the measurements made on the cable has its results exposed in Table VI. In this case, the DCT has P D = 1% for L q =16 and 32. These are indeed, the window sizes that presented the best results for all transforms. In general, it can be stated that for the first part of the spectrum, for both the measurements made on the antenna and on the cable, the window size L q =128 shows the best

5 TABELA V DETECTIO RATES FOR THE MEASUREMETS MADE O THE ATEA (%) - SUB-BAD#3. DFT MCLT DHT DCT DST TABELA VI SUB-BAD#3. DFT MCLT DHT DCT DST results on most of the measured data, which is plausible if it is analyzed that this part has signals with proportionally wider bandwidths, and consequently need detection windows with higher bandwidth. In the second sub-band, the L q that presented best results can not be identified, due to the variability of bandwidths found in different measurements. In the third sub-band, the window sizes that present best results are L q =16 and 32. The reason for this lies in the fact that the measured signals on this sub-band by both sensors are narrowband, and large windows cannot capture them properly. Tables VII and VIII present the detection rates for measurements for the whole spectrum (see Figure 2-B) using antenna and cable, respectively. On the former results, it can be noticed that the higher detection rates occur when data are obtained from the DHT and DST transforms and window sizes 256 and 128, while on the latter results, the higher detection rate occurs when the DST and DCT transforms and L q =16. On both Tables, we can also observe that the DFT presents the worst detection rates on both scenarios. Also, Tables VII and VIII reveal that the attained results are lower than those obtained with the frequency division suggested in Figure 2-A (see Tables I-VI). TABELA VII DETECTIO RATES FOR THE MEASUREMETS MADE O THE ATEA (%) - WHOLE SPECTRUM. DFT MCLT DHT DCT DST The best detection rates were obtained for the measurements made on both cable and antenna. Then, we can not state, based on our work, which sensor can provide the best performance. VI. COCLUDIG REMARKS This paper discussed and investigated some relevant aspects related to spectrum sensing technique for powering PLC TABELA VIII WHOLE SPECTRUM. DFT MCLT DHT DCT DST systems. Also, it introduced a spectrum sensing technique and, based on several assumptions and constraints, analyzed its performance based on measured data. Overall, the attained results indicate that the proposed technique is effective for spectrum sensing and gives some directions for the design of cognitive PLC systems. Finally, it can be concluded that the use of the spectrum by PLC systems can be enhanced by applying spectrum sensing techniques, such as the one discussed in this contribution. REFERÊCIAS [1] S. Galli, A. Scaglione, and Z. Wang, For the grid and through the grid: The role of power line communications in the smart grid, Proceedings of the IEEE, vol. 99, no. 6, pp , June 211. [2] Y. R. Ferreira, Inspection method on measuring unwanted emissions from broadband power line using mobile monitoring station, in IEEE International Symposium on Power Line Communications and Its Applications, Mar. 21, pp [3] B. Praho, M. Tlich, P. Pagani, A. Zeddam, and F. ouvel, Cognitive detection method of radio frequencies on power line networks, in IEEE International Symposium on Power Line Communications and Its Applications, 21. [4] P. Pagani, R. Razafferson, A. Zeddam, B. Praho, M. Tlich, J. Baudais, A. Maiga, O. Isson, G. Mijic, K. Kriznar, and S. Drakul, Electro magnetic compatibility for power line communications, in IEEE 21st International Symposium on Personal Indoor and Mobile Radio Communications, 21, pp [5] S. W. Oh, Y. L. Chiu, K.. g, R. Mo, Y. Ma, Y. Zeng, and A. A. S. aveen, Cognitive power line communication system for multiple channel access, in IEEE International Symposium on Power Line Communications and Its Applications, Apr. 29, pp [6] L. R. Amado, Uma contribuição à análise de técnicas de monitoramento de espectro para sistemas plc, M.S. thesis, Universidade Federal de Juiz de Fora, Aug [7] I. S. Areni, S. Tsuzuki, and Y. Yamada, A study of radiation detection methods for cognitive plc system, in IEEE International Symposium on Power Line Communications and Its Applications, 211, pp [8]. Weling, Feasibility study on detecting short wave radio stations on the powerlines for dynamic psd reduction as method for cognitive plc, in IEEE International Symposium on Power Line Communications and Its Applications, Apr. 211, pp [9] S. Haykin, D. J. Thomson, and J. H. Reed, Spectrum sensing for cognitive radio, in Proceedings of the IEEE, May 29, vol. 97, pp [1] H. S. Malvar, Signal processing with lapped transforms, Artech House, [11] M. C. V. I. F. Akyildiz, W. Y. Lee, and S. Mohanty, ext generation/dynamic spectrum access/cognitive radio wireless networks: A survey, Computer etworks, vol. 5, no. 13, pp , Sep. 26. [12] M. V. Ribeiro, C. A. G. Marques, C. A. Duque, A. S. Cerqueira, and J. L. R. Pereira, Detection of Disturbances in Voltage Signals for Power Quality Analysis Using HOS, EURASIP Journal on Advances in Signal Processing, vol. 27, 27, ID 59786, 13 pages. [13] S. Theodoridis and K. Koutroumbas, Pattern Recognition, Elsevier, 2nd edition, 23. [14] S. Haykin, eural etworks - Comprehensive Foundation, Pearson Education, 2nd edition, [15] Gage, Razor compuscope 16XX, [16] PREMO, Capacitive Coupler P , 211.

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